Prefabricated trusses are often used in the construction of buildings because of their strength, reliability, low cost, and ease of use. An increase in the use of more complex and varied trusses, however, has created manufacturing problems and increased production times.
Trusses are generally assembled on a jigging table. Jigging tables typically have a plurality of adjustable stops, or pucks, for indicating the proper positions of the elements of a truss and for holding these elements in position until they can be permanently secured together. The pucks must be repositioned on the jig surface for each different truss. Computer programs generally calculate the position of the pucks from a reference line, such as the edge of the table. Conventionally, an operator would measure the positions of the pucks from the reference line, manually move and secure the pucks into the desired positions, place the truss elements on the table against the pucks, fasten them together, remove the completed truss, and then repeat. Due to great variation and complexity in modern truss designs, a significant amount of production time is spent resetting the positions of the pucks and there is a high likelihood of operator error. Various approaches have been developed to enhance this process.
One method that has been developed to increase production efficiency in truss assembly is laser projection. This approach projects the image of a desired truss in actual shape and size onto a jig table. The pucks of the jig table are then simply moved to their corresponding locations as indicated by the laser projection. This minimizes or eliminates the measurement time needed with conventional systems and ensures accurate placement of the pucks. Known laser truss assembly systems are disclosed in U.S. Pat. No. 5,430,662 to Ahonen, U.S. Pat. No. 6,317,980 to Buck and U.S. Pat. No. 6,170,163 to Bordignon et al, which are hereby incorporated by reference. However, these types of systems do not eliminate the need to repeatedly secure and loosen the pucks for each truss design. Although effective in increasing the correctness of assembled trusses, the time it takes for an operator to manually position the pucks with their corresponding projected image is significant.
Another approach employs a system that automatically moves the pucks along the surface of the jig. Such systems are disclosed in U.S. Pat. No. 5,854,747 to Fairlie, U.S. Pat. No. 6,712,347 to Fredrickson et al, and U.S. Pat. No. 5,342,030 to Taylor, which are hereby incorporated by reference. The goal of such systems is speed and efficiency greater than prior systems such as manual jig tables and laser projection. For example, the '347 patent criticizes prior laser projection systems as being too slow and expensive. While these systems may speed up the process, they tend to suffer reliability and consistency issues. Because trusses are often made from wood, sawdust and wood chips often pile up on the jigging table. This debris can fall into the slots in which the pucks move, hampering or preventing the pucks from reaching their proper position or preventing the pucks from being properly secured. An operator assembling a truss based on faulty positioning caused by one of these problems may fail to notice when one of the pucks is not in its proper place, possibly leading to an entire batch of improperly aligned trusses. In addition, any error by the software or hardware system controlling the pucks is not likely to be caught by an operator as there is nothing to indicate that there are pucks that are not properly aligned. Moreover, although speed and efficiency can be increased with use of such a system, it often requires a large initial investment to completely replace all existing manual equipment for the automated equipment and a significant prior capital expenditure is wasted in discarding the previously used tables.
In existing systems, whether each individual puck is positioned on the top of the truss, the bottom of the truss, or in the interior of the truss is typically chosen essentially at random or in a pre-determined fashion not dependent on the shape of the particular truss, such as alternating between the top and the bottom. This can lead to truss members not being properly supported during assembly of trusses, which can decrease the efficiency and quality of assembly. Accordingly, it would be desirable to have an automated truss assembly system that positions the pucks on a truss assembly table to provide optimum truss support.